1. Technical Field
The present invention relates to a DC motor, and in particular, to a control apparatus controlling the number of rotations of a brushless DC motor and a control method thereof.
2. Description of the Related Art
The motor is a kind of energy conversion device, which converts electric energy to rotational or linear kinetic energy by means of electromagnetic phenomena. With the expansion of its fields of application due to the widespread use of electronic equipment in the electric, electronic, and mechanical industries, the importance of the motor is increasing as a critical driving source. In addition, with a rapid growth of the industries have been introduced the driving motors with faster speeds and larger capacities.
In particular, a precision control motor provides accurate movement with a fast response time and a wide range of speed controls. This motor operates according to control signals by repeating stop, start, and reverse actions, etc. The precision control motors are continually increasing in importance with the advance of technology and expansion of application fields such as advances in power electronics, advances in microcomputers, practical application of precision technology and high performance permanent magnets, and advances in surface mounting technology.
Examples of such precision control motors include stepping motors and brushless DC motors, which usually use highly efficient permanent magnets. The stepping motor is most often used for position controlling. In accurately controlling mechanical displacements, the stepping motors are suitable for micoms because they allow a digitalized control by using pulses. The stepping motor can perform a rotation of a specified angle and can stop with a high level of precision without a feedback for detecting the position of the motor shaft. Also, the stepping motor allows an open loop control, is easily controlled through digital signals, and has a maintenance torque when stopping.
However, the torque is low in the stepping motors, so that it is difficult to apply in fields requiring a high torque. Also, the stepping motor is likely to vibrate and resonate at certain frequencies, is weak against loads with inertia, and is likely to be stepped out during high-speed operations. Further, since a sufficient current cannot flow through winding wires due to an inductance effect of the winding wires during operation with a common driver, the torque is reduced with increasing pulse rate to yield a lower efficiency as compared to a DC motor.
Thus, a brushless DC motor, which provides a high torque, is preferred in controlling the position or the number of rotations. Not only is the brushless DC motor durable and efficient, it allows for easy use in a constant speed control and a variable speed control.
The brushless DC motor is devised without a brush, which acts as a commutator, while maintaining the properties of a DC motor, and can be classified as a sensor type or a sensorless type according to whether or not it has a sensor which detects both the position of the rotor and the rotary speed.
In Korean patent publication no. 1998-0013970, “sensorless BLDC motor position control method” is disclosed. It relates to a BLDC motor, wherein the counter electromotive forces generated in the stator are detected by the rotor of the motor, and pulses are divided into a multiple number, as these counter electromotive forces are compared with a reference voltage. The pulses are counted to determine the accurate position of the rotor of the motor with regard to the normal position of the rotor of the motor, and corresponding control signals are outputted to provide accurate position control.
This positioning control based on the control signals aims to control a rotational position of the rotor of the motor, but cannot control a rotational angle of the rotor.